Re: Is Fragmentation at IP layer even needed ?

[Warren Kumari <[e-mail]>]

On Mon, Feb 8, 2016 at 9:05 AM David Borman <[e-mail]> wrote:

>
> > On Feb 8, 2016, at 10:08 AM, Alexey Eromenko <[e-mail]> wrote:
> >
> > On Mon, Feb 8, 2016 at 5:52 PM, John Levine <[e-mail]> wrote:
> > >2. What kind of UDP applications use such big packets, over 1280 bytes ?
> >
> > >Um, the DNS with EDNS0, and particularly with DNSSEC?  Is this a trick
> question?
> >
> >
> > Yes.
> > The trick lies right there in IPv6 specification:
> >
> > "A node must be able to accept a fragmented packet that, after
> >    reassembly, is as large as 1500 octets."
> > ... which is not much greater than 1280 bytes defined as minimum MTU for
> IPv6.
> >
> > Basically, Fragmentation, as specified in RFC-2460 (IPv6), solves the
> problem ONLY for packet-sized between 1280 and 1500 bytes. Which is why I
> doubt if it is useful at all.
>
> That’s the minimum required implementation, which guarantees that you can
> send at least a basic ethernet sized packet to any host.  What is omitted
> in the above qoute is next 2 sentences:
>    "A node is permitted to
>    accept fragmented packets that reassemble to more than 1500 octets.
>    An upper-layer protocol or application that depends on IPv6
>    fragmentation to send packets larger than the MTU of a path should
>    not send packets larger than 1500 octets unless it has assurance that
>    the destination is capable of reassembling packets of that larger
>    size.”
> So if you are writing an application that needs >1500 octets, use an IPv6
> implementation that supports >1500 octet fragmentation and reassembly.
>

... but as an application writer (or, basically anyone else), I have no
control over the "IPv6 implementation". Even if I'm in an environment where
I do control the OS / model of all devices, and I know they support >1500
octet, it seems like a bad idea to *rely* on that. Sometime I'm going to
want to change OS / add some other device, be able to interact with some
other system. This sounds like vendor lock at its worst...

W




>
>                 -David Borman
>
> >
> > (I also understand, that IPv4 fragmentation is much more flexible, yes.
> It can go as small as 68 byte packets.)
>
>